IN A NUTSHELL:Because radiation normally present in space at lunar distances from Earth is comparable to the radiation received by the astronauts on the International Space Station, who stay in space up to six months at a time. A round trip to the Moon lasted no more than twelve days.

THE DETAILS: It is often claimed that the lethal radiation of deep space would have killed the Apollo astronauts who went to the Moon, since they spent several days outside of the safety of Earth’s protective magnetic field, which provides a shield against this radiation.

However, the claim’s premise is factually incorrect: on Earth we’re protected against deep space radiation mainly by the atmosphere, not by the planet’s magnetic field, which has a small role in shielding us.

The dose of cosmic radiation (ions traveling at nearly the speed of light) that reaches anyone who lives at sea level is approximately 0.3 millisieverts/year, which is the equivalent of a couple of chest X-rays. This rises to 0.8-1.2 millisieverts/year for people living at high altitudes, for example on a 3,000-meter (10,000-ft) mountain range. At 12,000 meters (40,000 feet), the usual altitude of airline flights, cosmic radiation rises further to 28 millisieverts/year: nearly a hundred times more than at sea level, even though the aircraft’s occupants are still well within the Earth’s magnetic field.

Once you leave the atmosphere, this radiation increases considerably right away. In low earth orbit, such as on the International Space Station, it averages 100 millisieverts/year. At this altitude, the protective effect of the Earth’s magnetic field becomes significant, but only for astronauts who follow equatorial orbits; the ISS has a highly inclined orbit. In interplanetary space the dose is 130-250 millisieverts/year and by some estimates may be as high as 800 millisieverts/year on a trip to Mars; on the surface of the Moon it drops to 70-120 millisieverts/year.*

* Shielding Space Travelers, Eugene N. Parker, emeritus physics professor at the University of Chicago and member of the National Academy of Sciences, in Scientific American, March 2006.

In other words, the doses of deep space radiation to which the Apollo vehicles were exposed are comparable to those that affect the International Space Station, yet the occupants of the ISS stay in space for up to six months without dying of radiation exposure, compared to a maximum of twelve days for the lunar astronauts during Apollo 17.